1. Field of the Invention
This invention relates to transfer molding methods used to encapsulate semiconductor, electrical and optical devices ("device") with epoxy or other thermoset resin formulations ("formulation").
2. Description of the Prior Art
Transfer molding involves inserting a preform tablet of formulation in a pot, heating the preform, either prior to or after inserting the tablet in the pot, to soften the formulation, using a plunger to transfer the softened formulation from the pot into device cavities in metal molds. The plunger is used to force the soft encapsulating composition along runners into a package cavity in which the device has previously been inserted. The molten formulation surrounds the device in each cavity and encapsulates it.
The formulation which constitutes the preform tablets comprises abrasive fillers and mold release agents which cause problems insofar as the metal molds are subjected to wear and to "staining."
A method is described in Semiconductor Packaging Update, 1994, Vol. 9, No. 6 (ISSN: 0889-9193) in which a special pencil-shaped, film wrapped preform, prepared according to Pas, U.S. Pat. No. 5,098,626, is employed, along with specialized molds which eliminate the conventional pots and runners. Film is drawn into the cavities by vacuum molding. That method has disadvantages in that it requires special preforms, and extensive, complex, and expensive retooling of the molding equipment.